Influenza epidemics exact a formidable toll on world health. Moreover, viral super-infections can produce antigenic shifting, resulting in more virulent pathogens (Monto, Clin Infect Dis 48 Suppl 1, S20-25, 2009). At present, the emergence of a novel influenza A H1N1 viral strain has created a pandemic, producing illness in over 70 countries. Additionally, the related avian influenza A viral strain, H5N1, represents a potentially catastrophic global health risk (Maines et al., Clin Infect Dis 48 Suppl 1, 520-25, 2008).
The influenza A viral genome encodes for 11 proteins and consists of eight segments of negative single-stranded RNA (Lamb and Krug, Orthomyxoviridae: The viruses and their replication., 4th edn, Philadelphia, Lippincott Williams and Wilkins, 2001). Each sub-genomic segment is coated by viral nucleoprotein (NP) and bound to a single viral RNA-dependent RNA-polymerase holoenzyme (RdRp), composed of PA, PB1 and PB2 subunits. Infection begins with the binding of the viral hemagglutinin (HA) protein to sialyated host cell surface glycoproteins (Chu and Whittaker, Proc Natl Acad Sci USA 101, 18153-18158, 2004; Skehel and Wiley, Am J Respir Crit. Care Med 152, S13-15, 1995). Following endocytosis, viral particles are trafficked through both early and late endosomes, with the intense acidification of the latter compartment altering the conformation of HA, leading to host-viral membrane fusion, and entry of the vRNPs into the cytosol (Sieczkarski and Whittaker, Traffic 4, 333-343, 2003). Nuclear localization signal sequences contained in NP, PB1 and/or PB2 are then bound by host cell karyopherins, and the vRNPs are transported though the nuclear pore complex (NPC, (Boulo et al., Virus Res 124, 12-21, 2007)).
Once in the nucleus, the RdRp commandeers 5′ caps from host mRNAs to prime transcription of viral mRNA (vmRNA, (Bouloy et al., Proc Natl Acad Sci USA 75, 4886-4890, 1978) (Engelhardt and Fodor, Rev Med Virol 16, 329-345, 2006)). After producing sufficient vmRNAs, the RdRp creates a positive sense template (cDNA), from which it synthesizes new viral genomes (vRNAs). The vRNAs are coated by NP and exported though the NPC by the viral factors M1 and NEP/NS2 (nuclear export protein) working in concert with the host nuclear export machinery. The viral envelope proteins HA, M2 and neuraminidase (NA) are translated on the rough endoplasmic reticulum (ER) and trafficked to the cell surface where they, along with the soluble factors M1, RdRp and eight distinct vRNPs, are packaged into budding virions.
To defend against this exploitation, the host mobilizes factors to confront the virus. IFNs orchestrate a large component of this anti-viral response, at both a cellular and organismal level (Grandvaux et al., Curr Opin Infect Dis 15, 259-267, 2002). To this end, over 2000 gene products are differentially regulated after IFN stimulation, including the important downstream anti-viral effectors MxA, PKR, RIG-I, and 2′S′-OAS (Grandvaux et al., Curr Opin Infect Dis 15, 259-267, 2002; Haller et al., Rev Sci Tech 28, 219-231., 2009; Nakhaei et al., Semin Immunol 21, 215-222, 2009). However, many viruses deploy anti-IFN countermeasures, which for influenza A virus are primarily enacted by the viral protein, NS1 (Hale et al., J Gen Virol 89, 2359-2376, 2008).